The present invention relates to a process for removing residual vinyl chloride monomer from an aqueous dispersion of a synthetic polymer. More specifically, it relates to a method for determining the residual vinyl chloride content of the dispersion during the monomer removal process.
Polymers of vinyl chloride may be obtained by polymerizing the constituent monomer or monomers using an aqueous emulsion, suspension, or microsuspension process. The resulting products, being in the form of an aqueous latex or suspension, i.e., an aqueous dispersion, invariably contain a significant quantity of residual monomer. In many instances, it is desirable to remove substantially all of the residual monomer from the dispersion, e.g., in those cases where the quality of the polymer is thereby improved, where the monomer merits recovery for further use, and/or where the presence of even minor quantities of residual monomer in the polymeric product would create problems in handling and use due to toxicological hazards. In cases where the monomer is a low boiling material and the polymerization has been carried out under autogenous pressure, much of the residual monomer remaining in the reaction vessel at the end of polymerization can be removed simply by venting the vessel. Nevertheless, the resulting dispersion will still contain an appreciable amount of residual monomer.
Conventionally, residual vinyl chloride monomer has been further removed from an aqueous dispersion after polymerization by heating and/or reducing the pressure within the reaction vessel, i.e., by subjecting the dispersion to boiling conditions. Since this method alone is not entirely acceptable for larger and deeper reaction vessels due to the difference in hydrostatic pressure between the top and bottom of the vessel and, accordingly, the non-uniform monomer removal rate therein, agitation and/or circulation within the reaction vessel have been suggested.
During such processes, it is necessary to monitor the content of vinyl chloride monomer in the dispersion so that the appropriate time for termination can be ascertained. Conventionally, such monitoring is accomplished by periodically sampling the dispersion and analyzing the sample by gas chromatography and/or infrared spectroscopy. Even though these monitoring techniques are highly accurate, they are attended by a number of significant problems. For example, such monitoring can cause an interruption in the removal process while the sample is being taken and analyzed and it has the potential for exposing an operator to contact with the dispersion while it is still in a toxicologically hazardous condition. Additionally, such monitoring inherently includes a time lag between sampling and termination of the process due to the period required for analyzing. Consequently, the decision to terminate is necessarily postponed beyond the time at which it could have been made. Such postponement is uneconomical and, further, it has the potential in some cases for causing polymer degradation due to the unnecessarily prolonged heat treatment of the dispersion.
Accordingly, it would be desirable to have a reliable method for monitoring the residual vinyl chloride content which would eliminate the possibility of operator contact, which would not cause an interruption in the removal process, and which could provide a nearly instantaneous indication of monomer content so that the decision to terminate could be made as soon as possible.